This invention relates to a system and method for measuring hearing, and more particularly to a system and method for measuring hearing that may be successfully applied to individuals unable to respond to instructions or request of a person administering the test. The system and method of testing herein described may therefore be applied to the measurement of hearing function in babies, for example, permitting assessment of their hearing at an earlier age than possible when methods or systems are used which depend on communication between tester and subject.
It has been known for hundreds of years that the simultaneous introduction to the ear of two single-frequency sinusoidal tones, known as primary tones, or simply as primaries, which are close both in frequency and in sound pressure level, results in the production of numerous audible intermodulation distortion products. The audible distortion products are caused by nonlinear processes within the ear which are, at the present time, of unknown origin. Typically, the frequencies of the primaries used are in the approximate ratio 1:1.2. It is usual to designate the lower in frequency of the two primaries as f.sub.l and the higher as f.sub.2. Of the intermodulation distortion generated by the ear, one in particular, with frequency 2f.sub.1 -f.sub.2, is normally perceptible to the subject to whom the tones are presented.
In 1979, Dr. David Kemp established that distortion tones produced by the cochlea (in the inner ear) could be detected and measured in the ears of normal-hearing persons by placing a sensitive microphone in the ear canal during presentation of the primaries. Subsequently, Kemp and his colleagues as well as numerous other researchers in various countries have obtained data demonstrating that the absence of measurable distortion tones is associated with hearing impairment in that region of the audible spectrum occupied by the two primary tones and the distortion tone. Such a test is often referred to in the literature as one employing the evoked distortion product (EDP) method.
Prior to his discovery of the physical and measurable character of auditory distortion, Kemp, in 1978, established the detectability of a nonlinear version of an impulsive signal returned, as he characterized it, in the form of a "reflection" from the inner ear, which appeared following a brief time interval after the application to the ear of an acoustic impulse. By applying a series of acoustic impulses to the ear and employing a method of averaging and other signal processing by which the linear components of the echo were cancelled, a practical technique for the assessment of the hearing function of passive subjects was developed. Kemp subsequently devised a system for measurement of the spectral and time-domain properties of the nonlinear echo, then made available to researchers a device to carry out such tests, made and sold by Otodynamics, Ltd. While the impulse/echo technique is not the only method used in research, it has gained wide use, especially in laboratory measurement of the hearing of infants and young children.
The present invention, while not using the impulse/echo method disclosed by Kemp in U.S. Pat. Nos. 4,374,526 and 4,884,447 may be regarded as an improvement over that method in several respects. The present invention, because it provides results more rapidly than the impulse/echo method, is more suitable for application to the screening of infants and young children for hearing impairment than methods based on the prior art, and is a useful technique in the research laboratory to be employed supplementary to such impulse/echo methods.
Referring to FIG. 1, in conducting the EDP test in accordance with the prior art, two primaries are presented to the ear, typically by two small transducers 12, analogous to miniature loudspeakers, each transducer presenting one of the two tones. An EDP is measured by placement of a sensitive microphone 14 in the ear canal 16 of the subject, and the output of the microphone is applied to the input of a spectrum or wave analyzer. Alternatively, the output of the microphone, after appropriate amplification, may be applied to the input terminals of an analog-to-digital converter for conversion to a binary-encoded representation of the output waveform of the microphone, and such representation analyzed by a digital computer program for determination of the spectrum of the microphone output signal.
The use of two transducers for presentation of the primaries is necessitated by the tendency of a single transducer, when multiple tones are applied to its input terminals in electrical form, to generate intermodulation distortion products as components of its acoustic output due to the nonlinear behavior of the transducer. Among these distortion components there are likely to be intermodulation distortion components at the same frequencies as those produced by the ear. The transducer's intermodulation distortion would interfere with the measurement of distortion produced by the ear.
A current limitation of the EDP method stems from the variability of emission measurements in normal-hearing ears. When one pair of primaries is presented to a subject, an EDP may not be detected or may be very low in level, leading to the conclusion by the tester that some impairment of the auditory system exists. In fact, the level of the EDP typically varies with frequency for any subject in a specific manner not predictable by any method now known. Consequently, measurement of one EDP alone may mislead the tester. Until now, the only ways of overcoming this problem were either to employ the impulse/echo method, which is relatively time-consuming and inefficient when compared to the EDP method, or to carry out the EDP method at a large number of frequencies in a sequential manner.
In prior art apparatus for measurement of the EDPs produced by pairs of primaries at frequencies spread over the audible frequency range, results are obtained by presenting one pair of primaries at a time and measuring a single EDP produced by that pair. Insofar as infants and small children tend to move and produce sounds that interfere with testing and produce results that are not usable, testing by use of the prior art must be extended for a period of time sufficient to obtain satisfactory data.
It is, therefore, a principal object of the present invention to provide a system and method for testing hearing that acquires information about hearing functionality simultaneously at multiple frequencies in contrast to the prior art EDP method which detects and measures a single EDP.
Another object of the present invention is to provide a system and method of testing hearing that eliminates the likelihood that when multiple pairs of primaries are employed in an EDP test, interfering intermodulation products will be caused by nonlinear interaction of the primaries radiated by each transducer, causing intermodulation products that would conceal or otherwise interfere with measurement of the EDP.
A further object of the invention is to provide a system and method for testing hearing that is more rapid than the prior art tests, so that much less time is taken to complete testing, thereby reducing the cost of each test without compromising the reliability of the data obtained.
Still another object of the invention is to provide a system and method for testing hearing that increases the reliability of tests made by the EDP method.
Yet a further object of the present invention is to provide a system and method of testing hearing that improves the efficiency of EDP testing by automatically limiting the duration of the EDP test to a time that is reasonable and sufficient for the determination of the functionality of the hearing of the subject.
Another object of the invention is to provide a method of testing hearing that facilitates the design of screening apparatus for testing the hearing of children, which apparatus may be conveniently operated by an individual with minimal training.